Modulation of Telomere Length
It has been recognized that telomeres, long chains of repeated nucleotides located at the tip of each chromosome, play a role in the protection and organization of the chromosome. In human cells, the sequence TTAGGG is repeated hundreds to thousands of times at both ends of every chromosome, depending on cell type and age. Harley, C. B. et al., Nature, 1990, 345, 458–460; Hastie, N. D. et al., Nature, 1990, 346,866–868. Telomeres also appear to have a role in cell aging which has broad implications for the study of aging and cell immortality that is manifested by cancerous cells.
Researchers have determined that telomere length is reduced whenever a cell divides and it has been suggested that telomere length controls the number of divisions before a cell's innate lifespan is spent. Harley, C. B. et al., Nature, 1990, 345, 458–460; Hastie, N. D. et al., Nature, 1990, 346,866–868. For example, normal human cells divide between 70–100 times and appear to lose about 50 nucleotides of their telomeres with each division. Some researchers have suggested that there is a strong correlation between telomere length and the aging of the entire human being. Greider, C. W., Curr. Opinion Cell Biol., 1991, 3, 444–451. Other studies have shown that telomeres undergo a dramatic transformation during the genesis and progression of cancer. Hastie, N. D. et al., Nature 1990, 346, 866–868. For example, it has been reported that when a cell becomes malignant, the telomeres become shortened with each cell division. Hastie, N. D. et al., Nature 1990, 346, 866–868. Experiments by Greider and Bacchetti and their colleagues have shown that at a very advanced and aggressive stage of tumor development, telomere shrinking may cease or even reverse. Counter, C. M. et al., EMBO J. 1992, 11, 1921–1929. It has been suggested, therefore, that telomere blockers may be useful for cancer therapy. In vitro studies have also shown that telomere length can be altered by electroporation of linearized vector containing human chromosome fragments into hybrid human-hamster cell lines. Chromosome fragments consisted of approximately 500 base pairs of the human telomeric repeat sequence TTAGGG and related variants such as TTGGGG, along with adjacent GC-rich repetitive sequences. Farr, C. et al., Proc. Natl. Acad. Sci. USA 1992, 88, 7006–7010. While this research suggests that telomere length affects cell division, no effective method for control of the aging process or cancer has been discovered. Therefore, there is an unmet need to identify effective modulators of telomere length.
Guanosine nucleotides, both as mononucleotides and in oligonucleotides or polynucleotides, are able to form arrays known as guanine quartets or G-quartets. For review, see Williamson, J. R., (1993) Curr. Opin. Struct. Biol. 3:357–362. G-quartets have been known for years, although interest has increased in the past several years because of their possible role in telomere structure and function.
In addition to their natural role (in telomeres, for example, though there may be others), oligonucleotides which have a GGGG motif or one or more GGG motifs are useful for inhibiting viral gene expression and viral growth and for inhibiting PLA2 enzyme activity and have long been believed to be useful as modulators of telomere length. Chemical modification of the oligonucleotides for such use is desirable and, in some cases, necessary for maximum activity.
It has now been clearly demonstrated both in vitro and in vivo that oligonucleotides containing a GGG motif are capable of modulating telomere length on mammalian chromosomes. Herbert et al., 1999, Proceedings Natl. Acad. Sci., USA, 96, 14276–14281.